1. Field of the Invention
This invention relates to a process for recovery of zinc products including essentially pure metal oxides and metals. The present invention more specifically relates to an improved method for reducing the formation of Zn(NH4)4Cl2 from ZnO/NH4Cl intermediate solutions during a recycling process for the recovery of essentially pure zinc oxide from EAF dust.
2. Description of Related Art
Zinc oxide typically is a fine white or grayish powder which has a variety of uses including as a rubber accelerator, as a pigment, as a dietary supplement and in the semiconductor field. Zinc oxide is found in commercial by-products including waste material streams such as fly ash and flue dust. Methods for recovering zinc oxides are known in the art, including recovering zinc oxide from industrial waste materials. Such previous methods have included leaching with mineral acid, caustic soda, ammonium hydroxide, and ammonium carbonate solutions. However, these methods have low yields of zinc oxide and typically do not recover pure zinc oxide, the recovered zinc oxide being contaminated with other metal salts. Therefore, in order to obtain pure zinc oxide, subsequent reduction and washing processes were necessary.
U.S. Pat. No. 3,849,121 to Burrows discloses a method for the selective recovery of zinc oxide from industrial waste. The Burrows method comprises leaching a waste material with an ammonium chloride solution at elevated temperatures, separating iron from solution, treating the solution with zinc metal and cooling the solution to precipitate zinc oxide. The Burrows patent discloses a method to take EAF dust which is mainly a mixture of iron and zinc oxides and, in a series of steps, to separate out the iron oxides and waste metals. However, the material obtained in the last step is a mixture of a small amount of zinc oxide, hydrated zinc phases which can include hydrates of zinc oxide and zinc hydroxide, as well as other phases and a large amount of diamino zinc dichloride Zn(NH3)2Cl2 or other similar compounds containing zinc and chlorine ions. Currently, the Burrows method is not economically viable because of Environmental Protection Agency guidelines established subsequent to the issuance of the Burrows patent. Additionally, the Burrows method is not a continuous method and, therefore, is not economical as a continuous process.
U.S. Pat. No. 4,071,357 to Peters discloses a method for recovering metal values which includes a steam distillation step and a calcining step to precipitate zinc carbonate and to convert the zinc carbonate to zinc oxide, respectively. Peters further discloses the use of a solution containing approximately equal amounts of ammonia and carbon dioxide to leach the flue dust at room temperature, resulting in the extraction of only about half of the zinc in the dust, almost 7% of the iron, less than 5% of the lead, and less than half of the cadmium.
Steam distillation is contrary to dilution. Steam distillation precipitates zinc carbonate, other carbonates and iron impurities. Steam distillation also disadvantageously results in an increase in temperature which drives off ammonia and carbon dioxide, resulting in the precipitation of iron impurities and then zinc carbonate and other dissolved metals. The purity of the zinc carbonate obtained depends on the rate of steam distillation and the efficiency of solids separation as a function of time. Calcining converts the zinc carbonate to zinc oxide, whereas washing and drying at temperatures between 100xc2x0 C. and 200xc2x0 C. converts the zinc compounds to zinc oxide.
U.S. Pat. No. 5,464,596 to Myerson, commonly assigned with the present application, discloses a method for the recovery of zinc oxide by treating a waste stream with a 23% ammonium chloride at 90xc2x0 C., separating undissolved components from the solution, displacing undesired metal ions from the solution using zinc metal, cooling the solution to precipitated out zinc compounds, washing the precipitate to remove various soluble zinc compounds, leaving zinc oxide of greater than 99%. The ""596 patent teaches that ammonium chloride solutions must be at least 90xc2x0 C. to sufficiently dissolve the zinc compounds. Heating an aqueous solution to such a temperature requires the expenditure of large amounts of energy. It is further taught that while NH4Cl concentrations below 23% do not dissolve the maximum amount of zinc oxide from the waste material, concentrations greater than 23% result in an impure zinc oxide due to the tendency of the NH4Cl to precipitate out of solution with the zinc compounds at such high concentrations. Furthermore, the cooling of the product solution results in the precipitation of various zinc species, resulting in crystallization of some species. Because of this, using the cooling step disclosed in Myerson ""596, one cannot use concentrations of ammonium chloride solutions above about 23%, limiting the usefulness of the process disclosed in Myerson ""596. Further, contaminates of the zinc oxide must be removed by an additional washing step.
U.S. Pat. No. 5,759,503 to Myerson, et al., commonly assigned with the present application, discloses a method for the recovery of zinc oxide by dissolving zinc oxide in an intermediate, diluting the intermediate by a factor of 3 to 30 by adding 70-1xc2x0 C. water, and filtering out the resultant zinc oxide crystals. The ""503 patent, along with its family of patents, disclose using ammonium chloride solutions of 23% and teach that using higher concentration ammonium chloride solutions will produce undesired results. This was the understanding at the time of invention of the processes disclosed and claimed in the Myerson ""503 patent and its family of patents. These undesired results include the precipitation of various zinc species, resulting in crystallization of some species. Because of this, using the cooling step disclosed in Myerson ""596, one cannot use concentrations of ammonium chloride solutions above about 23%, limiting the usefulness of the process disclosed in Myerson ""503 Although the processes disclosed and claimed in Myerson ""503 patent are valuable and perform admirably, it has now been discovered that by altering the process steps, including the addition of a dilution step, ammonium chloride solutions of 30% or greater can be used to recycle industrial waste streams.
Therefore, there exists a need for an alternative method that will recover essentially pure zinc oxide from industrial waste materials that is economical, quick, and efficient and, optionally also will allow the recovery of elemental lead, cadmium, and copper from industrial waste materials, at the lower end of the temperature range and at higher leach solution concentrations than previously thought possible.
The present invention satisfies these needs in a method which recovers essentially pure zinc oxide from waste material containing zinc or zinc oxide. Along with the essentially pure zinc oxide, zinc metal also can be recovered, as well as values of other metallic elements originally contained in the waste material, such as lead, silver, and cadmium. The solutions used in the process are recycled such that liquid wastes are kept to a minimum and, ideally, eliminated. The solids recovered from the process, namely, the zinc oxide, zinc metal, other chemical and metal values, and other residues all can be used in other processes. One such residue, an iron oxide cake, is of such a quality that it can be used directly as the feedstock for the typical steel production process.
Briefly, the present invention provides a method for recovery of high purity zinc oxide products from industrial waste streams such as electric arc furnace effluents (dusts, fumes, and vapors) containing zinc compounds comprising the steps of: (a) leaching the waste stream with a solution of 30% or greater by weight ammonium chloride at least about 70xc2x0 C., resulting in a first product solution and undissolved materials; (b) adding zinc metal to the first product solution in a cementation step, whereby zinc-displaceable metal ions contained in the first product solution are displaced by the zinc metal and precipitate out of the first product solution as metals, leaving a second product solution; and (c) diluting the second product solution with water, resulting in the precipitation of zinc oxide and a third product solution. The dilution step circumvents the crystallization of various species, allowing the use of the higher concentration ammonium chloride solution of approximately 30% or greater by weight. The various undissolved precipitates produced during the process comprise both waste products and chemical and metal values that can be recovered and sold, used in subsequent processes, or added to the feed to various industrial processes such as the iron and steel making processes.
The third product solution is concentrated after removing the zinc oxide, resulting in a fourth product solution comprising greater than 30% ammonium chloride. The fourth product solution then is combined with the original ammonium chloride solution of Step (a) to leach the industrial waste stream in a continuous process.
If the industrial waste material stream contains significant amounts of iron, the waste stream preferably is heated in a reducing atmosphere prior to leaching, resulting in an iron-containing residue and a combustion product (dust, fumes, and/or vapors) waste stream comprising oxides of zinc. The dust, fumes and/or vapors then are subjected to the process disclosed in this specification.
In yet another embodiment of the present invention, using a waste stream such as typical electric arc furnace dust (which typically contains zinc, cadmium, copper, lead, and iron metals and compounds) the precipitated metals will comprise zinc, lead, cadmium and copper compounds. This alternate embodiment further comprises the additional steps of: (1) treating the precipitated metals with an aqueous solution of either H2SO4 or NH4SO4, whereby zinc, cadmium, and copper compounds go into solution and lead compounds do not, resulting in a fifth product solution comprising zinc, cadmium and copper compounds and a second undissolved precipitate comprising lead compounds; (2) adding zinc metal to the fifth product solution, whereby cadmium and copper compounds are displaced by the zinc metal and precipitate out of the fifth product solution as a third precipitate, leaving a sixth product solution; and (3) treating the sixth product solution with calcium chloride, resulting in the precipitation of CaSO4 from the sixth product solution. Again, the undissolved precipitates produced during the process comprise both waste products and chemical and metal values that can be recovered and sold, used in subsequent processes, or added to the feed to various industrial processes such as the iron and steel making processes.
If it is desired to control sodium or potassium chloride concentrations during the process, additional steps of: (i) adjusting the pH of the second product solution to between about 5 and about 8 with NH4OH prior to step (c); (ii) cooling at least a portion of the second product solution to precipitate diamino zinc dichloride; (iii) evaporating the second product solution to precipitate NaCl and KCl; and (iv) combining the second product solution with the ammonium chloride solution to leach the waste stream.
Therefore, it is an object of the present invention to provide a method for recovering zinc oxide from waste materials, such as fly ash or flue dust, which contain other metals, such as iron oxide, lead oxide, cadmium, copper and other materials.
Yet another object of the present invention is to provide a method for recovering zinc oxide in which all leaching and washing solutions are recycled for further use, and no leaching or washing solutions are disposed of into the sewers or the environment.
Still another object of the present invention is to provide a method for recovering zinc oxide which also results in the precipitation in elemental form of any lead, cadmium and copper metals contained in the starting materials.
Another object of the present invention is to provide a method for recovering zinc metal, zinc oxide and/or iron oxide which is economical, quick and efficient.
These objects and other objects, features and advantages of the present invention will become apparent to one skilled in the art when the following Detailed Description of the Preferred Embodiments is read in conjunction with the attached figures.